spdx3: element_writer: switch from tab characters to two spaces

[stanislaw]

Hello everyone,

This is my first issue to this repository. I am new to SPDX and apologize for possible misunderstanding of the intentions behind the code I am modifying.

I am integrating the SPDX 3.0 export to the StrictDoc tool, see https://github.com/strictdoc-project/strictdoc/issues/1541.

The starting point for this issue is the fact that the Creation Information block is suddenly nested with a tab character when I try to populate an spdx3.*.SpdxDocument and write to a string buffer. My IDE is set up to ignore tab characters, so the output is displayed as if the indentation level was 1 character wide.

In this MR, the creation info is unindented, and a test is added to demonstrate the behavior I would expect to see.